The makers of portable audio devices, including those of cellular telephones and portable music devices, are increasingly adding functionality to their devices. For example, cellular telephones include features such as music playback systems, multimedia playback systems, video cameras, video streaming, two-way video calling, internet browsing, and other audio processing features. While there is a trend toward the inclusion of more features and improvements for current features, there is also a trend toward smaller portable audio devices. As the devices have become increasingly smaller, loudspeakers of the devices are smaller as well. However, speaker quality, at least in part, remains a function of size.
The desired or required size of a portable audio device may also limit the manufacturer's choices for speaker cavity size and the location of the speaker within the housing of the device. The efficiency of a loudspeaker can depend to some extent on the acoustic load placed on it by the way it couples to its surrounding structures such as the speaker cavity and/or the device housing. In electrical terms a load is something that dissipates power and does some work. In acoustic terms a load is an acoustic structure that has a particular acoustic impedance. Such an impedance may be that of radiation from an aperture into air. The frequency response of a loudspeaker system can depend upon how the system is “loaded” in much the same way that the output from a power amplifier depends on the load impedance. The power amplifier drives an electrical load specified in Ohms. A loudspeaker drives an acoustic load that may be specified in units of acoustic Ohms or Rayls. Introducing a speaker system to a particular load will produce a particular transfer function or frequency response and a sound pressure level (SPL) in decibels (dB) when the speaker is driven at a nominal input voltage or power, for example, 2.83V or 1 W input.
A portable audio device architecture typically includes a speaker or speakers that radiate sound energy out of a surface of the device. While the quality of a small speaker for use during voice telephone calls may be sufficient, a user may find using the same speaker for music and multimedia playback systems inadequate. Loudness and the bass response of a loudspeaker system of a portable audio device may be particularly lacking. Improvement of the sound quality may be desired by users who in particular use the device for music and/or multimedia playback.